In order to achieve higher peak data rates in future wireless networks, simultaneous transmission and reception of multiple carriers, referred to as carrier aggregation, is considered as a key element. The Third Generation Partnership Project (3GPP) has been standardizing the carrier aggregation for two technologies: High Speed Packet Access (HSPA), as described in a paper by K. Johansson, J. Bergman, D. Gerstenberger, M. Blomgren and A. Wallen, titled, “Multi-carrier HSPA evolution,” published in Proc. IEEE Vehicular Technology Conference (VTC), Barcelona, Spain, June 2009, the disclosure of which is incorporated herein by reference in its entirety; and Long Term Evolution (LTE), as described in a paper by G. Yuan, X. Zhang, Wang and Y. Yang, titled “Carrier aggregation for LTE-Advanced mobile communication systems,” published in IEEE Commun. Mag., vol. 48, no. 2, pp. 88-93, February 2010, the disclosure of which is also incorporated herein by reference in its entirety. From the perspective of mobile terminals, carrier aggregation poses unprecedented design challenges, especially when multiple, non-contiguous carriers need to be transmitted and received simultaneously.
Regarding the receiver architectures of mobile terminals, a direct conversion receiver is well suited for carrier aggregation. Although direct conversion receiver has gained much popularity recently, as indicated in the book, RF Microelectronics, by B. Razavi, Upper Saddle River, N.J., Prentice-Hall, 1998, the disclosure of which is incorporated herein by reference in its entirety, each carrier inevitably requires an individual receiver, thereby leading to inefficient implementation. On the other hand, a wideband IF double conversion receiver is known, as described in the paper by A. Springer, L. Maurer and R. Weigel, titled “RF system concepts for highly integrated RFICs for W-CDMA mobile radio terminals,” published in IEEE Trans. Microw. Theory and Tech., vol. 50, no. 1, pp. 254-267, January 2002, the disclosure of which is incorporated herein by reference in its entirety. The wideband IF double conversion receiver reuses both the RF mixing stage and IF mixing stage (i.e., local oscillators, or LOs, and mixers), allowing for cost-efficient and power-efficient implementation. Moreover, it retains many of the advantages of the direction conversion receiver, for example, highly-programmable channel selection essential to aggressive carrier aggregation.
The wideband IF double conversion receiver architecture may be applied to multi-carrier reception. In, e.g., a dual-carrier double conversion receiver, one receiver branch may receive and process a first carrier, while another receiver branch receives and processes a second carrier. By selecting appropriate LO frequencies and mixer parameters, the second receiver branch may share LOs and mixers with the first branch, to conserve hardware. One drawback of such a dual-carrier double conversion receiver is the sensitivity to imbalance between the In-phase (I) and Quadrature (Q) components of a received signal (known as IQ imbalance) stemming from the shared, gain- and phase-imbalanced LOs and mixers. The problems of IQ imbalance are elucidated in papers by J. C. Rudell, J.-J Ou, T. B. Cha, G. Chien, F. Brianti, J. A. Weldon and P. R. Gray, titled, “A 1.9-GHz wide-band IF double conversion CMOS receiver for cordless telephone applications,” published in IEEE J. of Solid-State Circuits, vol. 32, no. 12, pp. 2071-2088, December 1997, and S. Cho and H. S. Lee, titled, “Effect of phase mismatch on image rejection in Weaver architecture,” published in IEEE Trans. Microw. Wireless Comp. Letters, vol. 17, no. 1, pp. 70-72, January 2007, the disclosures of which are incorporated herein by reference in their entireties.
Coping with IQ imbalance is an area of much research. The remedies for IQ imbalance are largely categorized into two approaches: analog calibration assisted by digital control, and digital compensation. In analog calibration, a digital circuit measures the distortion and accordingly controls the RF/analog circuit (e.g., local oscillator) in the direction that minimizes distortion. This approach is described in papers by L. Der and B. Razavi, titled “A 2-GHz CMOS image-reject receiver with LMS calibration,” published in IEEE J. of Solid-State Circuits, vol. 38, no. 2, pp. 167-175, February 2003, and M. Valkama and M. Renfors, titled “A novel image rejection architecture for quadrature radio receivers,” published in IEEE Trans. Circuits Syst. II—Express Briefs, vol. 51, no. 2, pp. 61-68, February 2004, the disclosures of which are incorporated herein by reference in their entireties.
On the other hand, digital compensation cancels IQ imbalance in a purely digital fashion, i.e., without controlling RF/analog components. This approach is described in papers by A. Tarighat, R. Bagheri and A. Sayed, regarding Orthogonal Frequency Division Multiplexing (OFDM) systems, titled, “Compensation schemes and performance analysis of IQ imbalances in OFDM receivers,” published in IEEE Trans. Signal Processing, vol. 53, no. 8, pp. 3257-3268, August 2005, and Páter Kiss and V. Prodanov, titled, “One-tap wideband I/Q compensation for zero-IF filters,” published in IEEE Trans. Circuits Syst. I—Reg. Papers, vol. 51, no. 6, pp. 1062-1074, June 2004, the disclosures of which is incorporated herein by reference in their entireties. However, digital IQ imbalance compensation has not been studied in the context of carrier aggregation.